A bug in Google Apps' Sync for Microsoft Outlook that has apparently existed for months is causing some email messages to remain in Gmail servers and not be downloaded into Outlook, causing end-users to overlook messages or see them late.

End-users in organizations hit by the bug are having to check the Gmail Web interface periodically to make sure they aren't missing any email messages in their Outlook PC client.

Google acknowledged the bug on March 12 and promised a fix for early last week, but was unable to deliver it. The latest plan is to push out a solution by Wednesday this week.

"After we fix the server side problem, we will release an automatic update that will resynchronize your mailbox to help ensure that any mail that wasn’t downloaded gets downloaded," wrote a Google representative identified as Advisor Wesley in the Google Apps discussion forum on March 12.

Wesley said the bug had been introduced in an upgrade earlier in March, but affected Apps administrators have indicated in discussion forum threads that their organizations have had the problem for months, maybe as far back as July of last year.

It's not clear how widespread the problem is or if specific conditions trigger it. Wesley wrote that the bug "primarily" affects end-users who haven't downloaded messages to Outlook for several days straight. However, some Apps administrators challenge this assessment in the threads, saying the bug has hit end-users who haven't let messages accumulate in Gmail.

Google didn't immediately reply to a request for comment.

Google launched Apps Sync for Microsoft Outlook, a free Outlook plug-in, in June 2009 to give users the option to use Outlook as a front end to the server-side Gmail and Calendar components of the Premier and Education editions of the Apps hosted collaboration and communication suite. The Premier edition costs $50 per user per year, while the Education edition is free.

Google made a big splash when it launched Apps Sync for Microsoft Outlook, saying it would be an important piece in smoothing the way for large enterprises to switch from Exchange to Gmail.

Last week, Google launched a free server-side tool, called Google Apps Migration for Microsoft Exchange, to simplify and automate the migration of email, calendar, and contacts data from Exchange to Google Apps.